The invention relates to a method for manufacturing of strips of stainless steel, comprising rolling in cold condition of strips which in a foregoing process have been manufactured through strip casting and/or have been hot rolled. The invention also relates to an integrated rolling mill line to be used at the carrying out of the method.
Cold rolling of stainless steel strips is performed for one or several purposes. The basic purpose is generally to reduce the thickness of the starting strips, which normally have been hot rolled in a foregoing hot rolling line to a thickness of the hot rolled strips, which is not less than 1.5 mm and normally is in the order of 2-4 mm, but can be up to 6 mm. A main purpose or a secondary purpose of the cold rolling also may be to increase the strength of the strip material.
Usually, it is also a purpose of the treatment of the steel strip in the integrated rolling mill line to afford the strip certain surface features. The cold rolling, the annealing, and the pickling in this respect cooperate and have in different ways influence on the end result. It should in this connection be pointed out that the level of ambition as far as desired surfaces are concerned may vary very much. In some cases, a very fine, high gloss surface, a so called 2B-surface or finer, is desired. In other cases, a considerably more raw surface may be good enough, i.e. a beautifully pickled surface. The removal of scales, and pickling play an important role in this respect, whether the purpose is to produce a high gloss strip with a very fine surface, or a final product having that surface structure which is achieved after pickling but without subsequent skin-pass rolling, or other surface of good quality. It is particularly important that the scale residues can be easily removed without heavy blasting. The surface structure would generally be significantly impaired, if, e.g. a very powerful blasting would be required prior to the pickling.
Conventionally, initial annealing, cooling, and descaling through shot-blasting as well as pickling in one or more steps precede the cold rolling, for the achievement of a starting material for the cold rolling without oxides and scale residues from the foregoing hot rolling but often with defects because of powerful, scale-breaking shot blasting. As an alternative the hot rolling can completely or partly be replaced by manufacturing of strips through casting, which strips may have a thickness down to what is normal for hot rolled strips or be a few millimeters thicker, but also in this case the cold rolling normally is preceded by initial annealing, cooling, scale-breaking shot-blasting, and pickling, to the extent the technique has been implemented at all. At the cold rolling, which conventionally is carried out in a plurality of consecutive cold rolling operations, possibly alternating with annealing, cooling, descaling, and pickling operations, the thickness can be reduced down to 1 mm and in some cases to even thinner gauges. At the same time it is possible to produce, in these conventional cold rolling mills, strips with a very fine surface, a so called 2B-surface, if the rolling is finished by heat treatment, pickling, and skin-pass-rolling, or even finer if bright annealing is employed. Further is it knownxe2x80x94U.S. Pat. No. 5,197,179 and EP 0 837 147xe2x80x94to perform at least a first cold rolling operation on the cooled hot rolled strip or on the cooled cast strip prior to heat treatment, pickling, and possible further cold rolling operations in order to bring the strip to desired final gauge. It is, however, characteristic for methods and rolling mill lines known so far that they are expensive and/or difficult to adapt to widely disparate requirements as far as strip thickness, surface conditions, and strength of the final product are concerned. This particularly applies when hot rolling and subsequent cold rolling, as well as operations in connection with the hot rolling and the cold rolling, are considered as an integrated process of production.
It is a purpose of the invention to attack and solve the above complex of problems. More particularly, the invention aims at facilitating the removal of oxides and scales from the cast and/or hot rolled steel strip, in which process the pickling constitutes an integrated part, by a treatment of the cast and/or hot rolled stainless steel strip prior to descaling and pickling, which treatment is characteristic for the invention. The invention is, however, not connected to any particular pickling technique. Generally, any pickling method, which is suited for pickling of stainless steels, can be employed in the method and the production line according to the invention.
These and other objectives can be achieved therein that the cast and/or hot rolled strip, which is dark coloured by oxides on the surfaces of the strips, remaining from the foregoing manufacturing of the said cast and/or hot rolled strip, is cold rolled in one or more consecutive cold rolling passes reducing the strip thickness by 10-75% and crackling the oxide scales, i.e. so that cracks are produced in the oxide scales, that the strip then is annealed in a furnace having a furnace atmosphere obtainable by heating the furnace by means of a burner, which consumes a liquid or gaseous fuel, which is combusted by means of a gas which contains at least 85 vol-% oxygen and not more than 10 vol-% nitrogen, whereafter the strip is cooled and subjected to at least any descaling operation and is pickled.
The initial cold rolling of the strip, which is dark coloured by oxides on the surfaces of the strip, remaining from the foregoing manufacture of the cast and/or hot rolled stainless steel strip, can be considered as an initial descaling operation, which can facilitate the efficient descaling that is performed later, after the annealing, but before the strip is pickled. In order that the said initial crackling shall be possible to be utilised efficiently in order to facilitate later descaling and pickling it is desirable that it as far as possible is not eliminated in connection with the annealing, i.e. so that fissures or cracks in the oxide layers do not heal up at the annealing. This desirable effect is to a considerable degree achieved therein that the strips are annealed in the specific atmosphere of the annealing furnace, which contains max 10 vol-% oxygen, preferably max 6 vol-% oxygen, while the main part consists of carbon dioxide, steam and a minor amount of nitrogen, which substantially emanates from air that possibly may leak in. A furnace atmosphere of that type can be achieved, e.g. through the technique which is disclosed in WO95/24509, the content of which herewith is incorporated in this text by reference. In the atmosphere of the furnace, which is poor of oxygen, the strip can be annealed at a temperature of 1050-1200xc2x0 C. during such a long period of time that the strip will be through-heatened and be recrystallized without at the same time oxidising the metal surfaces, which are exposed because of the crackling, to an extent that it would make the subsequent descaling and pickling more difficult.
Different techniques of descaling can be employed without damaging the strip surfaces, because of the crackling of the scales in connection with the initial cold rolling of the strips in combination with the annealing in the furnace atmosphere that is poor of oxygen. Conventionally, descaling is carried out through powerful shot blasting in one or more steps, a treatment which however would cause the non-desired damages of the strip surfaces, if employed. According to an aspect of the invention, the descaling instead is carried out by bending the strip repeatedly in different directions about rolls, at the same time as the strip is cold-stretched, so that it is permanently elongated 2-10% prior to pickling according to a technique, which is known per se through EP 0 738 781. Through this treatment an efficient descaling is achieved without impairing the strip surfaces. This descaling can be completed by a mild blasting, which can be performed before or after the descaling, preferably before, aiming at removing only loose oxides in order, through accumulation of oxides, not do disturb subsequent descaling. If the blasting is carried out subsequent to the descaling it is correspondingly achieved that loose oxides are removed, the blasting in each case being carried out in such a mild way that the metallic surfaces of the strip are not impaired. Typically therefore, the descaling after annealing is completed through cold-stretching, wherein the strip is bent repeatedly about rolls, in combination with a gentle, not surface damaging, blasting before or after the cold stretching. Since the scales still are crackled after annealing and therefore easy to break, it is also conceivable to carry out the descaling through only a light blasting and brushing, or through cold stretching the strip plus brushing, or through only brushing.
Further characteristic features and aspects of the invention will be apparent from the appending claims and from the following detailed description of the invention. In this description will be explained how the invention can be employed in a number of different variants of rolling mill lines, in which the initial cold rolling of hot rolled strips or corresponding and the treatment of the strips between said initial cold rolling and pickling, as has been described in the foregoing is an integrated part. It should, however, be pointed out that the usability of the invention is not restricted to any of the described applications that can be used generally in connection with cold rolling of stainless steel strips.